1. Field of the Invention
The present invention relates to charge-coupled device (CCD) linear sensors.
2. Description of the Related Art
Image-reading units including CCD linear sensors are included in a variety of known electronic apparatuses, for example, image scanners, copying machines, and facsimile machines.
A CCD linear sensor includes a horizontal electronic shutter mechanism. For example, as disclosed in Japanese Unexamined Patent Application Publication No. 11-135766, a CCD linear sensor includes a sensor array including a row of sensing elements on a substrate, a horizontal transfer register parallel to the sensor array at a first side of the sensor array, read gates between the sensing elements and the horizontal transfer register, an electronic shutter drain parallel to the sensor array at a second side of the sensor array, and electronic shutter gates parallel to the sensor array between the sensor array and the electronic shutter drain.
In normal operation, electric charge built up in the sensing elements is transferred to an image-processing circuit through the read gates and the horizontal transfer register. In operation of electronic shutters, electric charge built up in the corresponding sensing element is discharged to the electronic shutter drain through the electronic shutter gates.
In this CCD linear sensor, the potential at the electronic shutter gates is kept higher than that at the read gates so that saturated charge storage capacity in each sensing element is determined by the potential at the electronic shutter gates. In this arrangement, when electric charge to be stored in each sensing element exceeds the saturated charge storage capacity, excess electric charge overflows into the electronic shutter drain through the electronic shutter gates.
That is, in the known CCD linear sensor including the horizontal electronic shutter mechanism, the electronic shutter gates also serve as overflow barriers that determine the saturated charge storage capacity in each sensing element.
However, this arrangement is likely to cause variance in output signals from the sensor array.
That is, in the known CCD linear sensor, since the electronic shutter gates extend in the shape of a narrow strip, it is likely that the electronic shutter gates are not accurately formed on a wafer by a semiconductor manufacturing process, for example, a lithographic step and an etching step. That is, the manufacturing error of the electronic shutter gates will probably occur. For example, the length of the electronic shutter gates and the impurity profile in the electronic shutter gates will vary. Thus, potentials of the electronic shutter gates are likely to vary for the individual sensing elements, and the saturated charge storage capacity varies from sensing element to sensing element, as schematically shown in FIG. 6. Consequently, variance occurs in output signals from the sensor array.